The present invention generally relates to rotation control apparatuses for constant linear velocity system information recording discs, and more particularly to a rotation control apparatus which controls a rotation frequency of a constant linear velocity system information recording disc so that a relative linear velocity between a pickup device and the information recording disc is always maintained constant.
When recording and reproducing an information signal on and from a constant linear velocity system information recording disc (hereinafter simply referred to as a CLV system disc), a rotation control is carried out so that a relative linear velocity between a pickup device and the disc is always constant. At the time of a reproduction, the relative linear velocity can always be maintained constant by carrying out the rotation control so that a value obtained by reproducing a signal which is pre-recorded on the disc and has a constant frequency becomes a regular value. For example, in the case where the information signal pre-recorded on the disc is a video signal, a horizontal synchronizing signal within the video signal is used as the signal having the constant frequency, and bit clock pulses are used as the signal having the constant frequency in the case where the information signal pre-recorded on the disc is a digital audio signal.
On the other hand, at the time of a recording, the signal having the constant frequency is not yet recorded on the disc and it is hence impossible to use such a signal for the rotation control. But the relative linear velocity is determined by a product of the length (circumference) of each track turn on the disc and the rotation frequency. Hence, it is possible to maintain the relative linear velocity constant by controlling the rotation frequency of the disc inversely proportional to a radius at a position on the disc. Conventionally, when recording the information signal from the inner periphery to the outer periphery of the disc, the relative linear velocity is maintained constant by presetting the rotation frequency at an innermost peripheral position where the recording is started and by gradually decreasing the rotation frequency with a predetermined rate proportionally to the radius at each position on the disc.
However, the conventional method of controlling the rotation of the disc at the time of the recording is only effective when continuously recording the information signal throughout the entire recording region on the disc from a recording start position to a recording end position, such as the case where the recording is carried out by a cutting apparatus. In the case where the disc is removed from the recording apparatus by discontinuing the recording of the information signal in the recording region and the recording is resumed after a predetermined time has elapsed by once again loading the disc on the recording apparatus so as to newly record an information signal from a position on the disc where the previous recording had been discontinued, it is impossible to control the relative linear speed according to the conventional method.